Effects of water stress on phosphorus transport to the xylem

Summary Tomato plants were treated for one hour in nutrient solutions at-10.4 atm. Roots were excised, transferred to solutions at-0.4 atm and put into a pressure chamber to induce rates of water flow similar to those in transpiring plants.For roots continuously at-0.4 atm, the xylem sap had much higher phosphorus concentrations than the external solution, which contained 6 p.p.m. phosphorus.Roots previously treated at-10.4 atm had much lower concentrations in the sylem sap than in the external solution and the amount of phosphorus transported and the water flow were linearly related. This phosphorus transport was due to passive movement as shown by measuring transport of both ³²P and ¹⁴C mannitol. Thus transport to the xylem mediated by active processes was abolished even though uptake by the roots remained substantial. These results obtained after plasmolysis support the view that radial transport to the xylem includes uptake into and movement through the symplast.